Bar or gym? If between an aperitif and a workout session the choice regularly falls on the former, perhaps it is not just a question of laziness. Those who snub physical effort should not feel absolved, but a team of scientists decided to get to the bottom of the matter and, investigating the dynamics that are triggered in the brain when we give in to the countless temptations that can prevent us from doing physical activity, they deciphered which chemical substance and which nerve cells are involved. The experts discovered that the chemical messenger orexin and the neurons that are part of this system mediate the decision between snacks and sports. The authors of the study published in ‘Nature Neuroscience’ – researchers from the ETH, the Swiss Federal Institute of Technology in Zurich – observed this mechanism in mice and believe that the results can be transferred to humans.
In the experiment, mice with a blocked orexin system preferred the smoothie offered to them more often and exercise less often. These neuroscientific foundations, the authors explain, are relevant because many people do not exercise enough. Most of us have probably already missed an appointment with fitness one or more times in favor of more comfortable alternative activities of daily life. According to the World Health Organization, 80% of adolescents and 27% of adults do not move enough. And obesity is increasing at an alarming rate not only among adults, but also among children and adolescents, experts remind.
“Despite these statistics, many people manage to resist the constantly present distractions and get enough exercise,” says Denis Burdakov, professor of neuroscience at ETH Zurich. “We wanted to know what helps us make these decisions in our brain.” In experiments with mice, researchers were able to show that orexin plays a key role in this process. It is one of more than a hundred messenger substances active in the brain. Other chemical messengers, such as serotonin and dopamine, were discovered a long time ago and their role has largely been decoded. The situation with orexin is different: researchers discovered it relatively late, about 25 years ago, and are now clarifying its functions step by step. Burdakov is one of the scientists who has devoted himself to this line of study.
“In neuroscience, dopamine is a popular explanation for why we choose to do some things but avoid others,” Burdakov says. This brain messenger is crucial to our overall motivation. “However, our current understanding of dopamine doesn’t easily explain why we choose to exercise instead of eat,” he continues. “Our brains release dopamine when we eat and when we exercise, which doesn’t explain why we choose one over the other.”
For this reason, the researchers designed a sophisticated behavioral experiment for mice. They could freely choose between eight different options, 10-minute trials, including a running wheel and a milkshake bar where they could enjoy a strawberry smoothie. “Mice love it for the same reason that people do: it contains a lot of sugar and fat and it tastes good,” Burdakov emphasizes. In the test, the scientists therefore compared different groups of mice: one composed of normal mice and one of rodents in which the orexin systems had been blocked by drugs or genetic modification of their cells. Mice with an intact orexin system spent twice as much time on the running wheel and half as much time at the milkshake bar as mice whose orexin system had been blocked.
But the scientists note that the behavior of the two groups did not differ in experiments in which the scientists offered the mice only the running wheel or only the smoothie. “This means that the primary role of the orexin system is not to control how much the mice move or how much they eat,” Burdakov says. “Rather, it seems to be crucial for making the decision between one and the other, when both options are available.” Without orexin, the decision was strongly in favor of the smoothie, and the mice forwent exercise in favor of eating. “Now it’s a matter of verifying our results in humans,” says Daria Peleg-Raibstein, a group leader at ETH Zurich who led the study with Burdakov. This could be done, they say, by examining people who have a genetically restricted orexin system – there are about one in 2,000 of them – that is, patients with the sleep disorder narcolepsy. Another option would be to use a drug that blocks orexin (it is approved for insomnia). “If we understand how the brain arbitrates between food consumption and physical activity, we can develop more effective strategies to address the global obesity epidemic and related metabolic disorders,” Peleg-Raibstein concludes, referring to possible interventions to help overcome barriers to exercise.
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